Structural Effects of Creep and Age-Adjusted Effective Modulus Method
We begin with creep analysis of structures having homogeneous material properties, which is a feature requiring uniformity of concrete age and, in the case of drying exposure, also uniformity of concrete wall thickness. We explain the Boltzmann–Volterra elastic–viscoelastic analogy for creep with a linear stress dependence and discuss the effects of changes in structural system, which occur commonly during modern construction. Then, we derive and expound in detail the age-adjusted effective modulus method (AAEM), the simplest method to obtain good approximate estimates of the creep effects in the presence of aging—a method that is featured by now in most, if not all, design codes and recommendations. Our discussion then proceeds to calculating the stress redistributions in structures, and cross sections in which the creep properties are not homogeneous, or in which the nonuniformity is introduced by drying. Careful attention is given to stress relaxation in prestressed members, which is caused not only by the creep and shrinkage of concrete but also by the relaxation of prestressing steel at variable stress and temperature. We present a recently developed viscoplastic model for prestressing steel, justify it mathematically, emphasize that the tendon relaxation should be computed as part of creep structural analysis, and point out that daily cycles of tendon heating can greatly increase the prestress loss. We finish our discussion with the creep buckling of columns and shells, accurate assessment of which is particularly important for structural safety.